1. Field of the Invention
The present invention relates to fabrication processes and structures for quantum dots, and in particular, for semiconductor quantum dots lattices and the process for making same.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by reference numbers enclosed in brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the Section entitled xe2x80x9cReferencesxe2x80x9d in the Detailed Description of the Preferred Embodiment. Each of these publications is incorporated by reference herein.)
Self-assembled semiconductor quantum dots (QDs) [1-7] provide a convenient means of exploring the physics of zero-dimensional (0D) quantum-confined systems. The size of QDs is on the order of an electron wavelength and carrier energy levels are quantized. The sequential loading of electrons and holes and the three-dimensional (3D) confinement character of the carriers in the QDs have been previously demonstrated [7-9]. The sharp density of states in the QDs yields, as expected, ultra narrow luminescence lines and several studies have recently shown the importance and complexity of many-body effects in the relaxation processes involved in strongly excited quantum dots [10-13]. In addition to their xe2x80x98atom-likexe2x80x99 properties, the potential application of QDs has been explored in a wide variety of novel devices [14-18].
Moving from random arrays of xe2x80x9catom-likexe2x80x9d QDs to a QD lattice would offer the possibility of novel and unexpected properties that are tied to electronic or photonic QD coupling within an array of QDs. Ideally, the QD lattice should have long-range order, as well as a controllable crystal structure with an adjustable number of QDs in the basis. These ideal properties have not yet been achieved, and usually when discussing properties of self-assembled QDs, one deals either with an isolated QD or an ensemble of QDs that are randomly distributed on a plane within a structure. In this case, electronic coupling is not controlled and is dependent on the QDs"" density.
This invention describes a method for producing QD lattices with a wide variety of dimensions and orientation for the unit cell. The technique described can be applied to a wide variety of semiconductor or metal quantum dots deposited by epitaxy.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method for growing strain-engineered, self-assembled, semiconductor quantum dots (QDs) into ordered lattices. The nucleation and positioning of QDs into lattices is achieved using a periodic sub-surface lattice of stressor points and a spacer layer built-up on a substrate. Three different embodiments of two-dimensional (2D) square lattices are described herein. The unit cell dimensions, orientation and the number of QDs in the ordered lattice are tunable. Moreover, a 2D lattice can be replicated at periodic intervals along the growth direction to form a three-dimensional (3D) lattice of QDs.